1. Field of the Invention
This invention relates to a process for producing a one-side electrogalvanized steel sheet and particularly to a posttreatment process for improving the susceptibility of the non-electrogalvanized side of one-side electrogalvanized steel sheet to a dip-type phosphate salt treatment, a spray-type phosphate salt treatment or the like.
In the present invention, a one-side electrogalvanized steel sheet means a steel sheet whose one side is electrogalvanized with zinc or alloy composed mainly of zinc or whose one side is subjected to a dispersive or composite plating, and concretely it means a steel sheet whose one side is electrogalvanized with a zinc system such as Zn-Ni system, Zn-Ni-Co system, Zn-Fe system, Zn-Ni-Fe system, Zn-Ni-Fe-Cr system, Zn-Mn system, Zn-Mg system, Zn-Bi system, Zn-Cr system, Zn-Ni-Cr system, Zn-Ni-SiO.sub.2 system, Zn-Ni-Cr-SiO.sub.2 system or Zn-Ni-BaCrO.sub.4 system.
2. Prior Art
One-side electrogalvanized steel sheets are used mainly as outer casings for automobiles, and generally it is a usual practice to apply a phosphate salt treatment to the non-electrogalvanized side of a one-side electrogalvanized steel sheet as a coating treatment before coating of the non-electrogalvanized side with a paint.
In the phosphate salt treatment, it is necessary to clean the non-electrogalvanized side in a well known manner to bring the non-electrogalvanized side into such a state as not to prevent nuclear generation and growth of crystals in chemical conversion coating. However, the non-electrogalvanized side of the one-side electrogalvanized steel sheet is corroded with the galvanizing solution in the process for one-side electrogalvanization and as a result the corrosion products formed on the surface inhibit the nuclear generation, etc. of crystals in the chemical conversion coating. In order to obtain a good chemical conversion coatability on the non-electrogalvanized side, it is thus necessary to prevent the formation of the corrosion products or remove the formed corrosion products.
The following procedures have been proposed to solve these problems.
A process for removing corrosion products formed on the non-electrogalvanized side by a mechanical means, for example, by grinding with a brush, etc. after the one-side electrogalvanization [Japanese Patent Application Kokai (Laid-open) Nos. 59-126788 and 60-43499], where the non-electrogalvanized side has scars due to the grinding with a brush and there is still a problem on the appearance.
A process for treating the non-electrogalvanized side with an aqueous oxalic acid solution after the one-side electrogalvanization [Japanese Patent Application Kokai (Laid-open) No.60-200974], where the process is effective for preventing the non-electrogalvanized side from discoloration, but fails to remove a trace amount of galvanization metals such as Zn, etc. deposided on the non-electrogalvanized side during the one-side electrogalvanization, and thus the improvement effect on the susceptibility to the phosphate salt treatment is not satisfactory.
A process for applying a peeling treatment to the non-electrogalvanized side by electrolysis using the non-electrogalvanized side as an anode, for example, using the non-electrogalvanized side of one-side electrogalvanized steel sheet 1 as anode in an electrolytic treatment tank 9 as shown in FIG. 2, after the one-side electrogalvanization [Japanese Patent Application Kokai (Laid open) Nos. 58-133395 and 59-96292]. In FIG. 2, the non-electrogalvanized side of one-side electrogalvanized steel sheet is counterposed to an electrode 15, numeral 16 is press rolls and numeral 17 is electroconductive rolls. The process for applying the peeling treatment is satisfactory in that a good chemical conversion coating is obtained in a spray-type chemical conversion treatment which has been hitherto used by automobile makers. However, it is not satisfactory in a dip-type chemical conversion treatment which has been recently increased to a great degree. That is, since the process for applying the peeling treatment does not specify a combination of the concentration of agent, which is necessary for ensuring the electroconductivity in the treating bath, and pH-controlling agent, the performance of the treating bath is unstable in peeling by electrolysis. This results in problems such that the process cannot produce a steel sheet with a good appearance of the peeled surface and a good performance in the dip-type chemical conversion treatment.
A process for applying a peeling treatment to the non-electrogalvanized side by electrolysis using the non-electrogalvanized side as an anode in a phosphoric acid-based bath after the one-side electrogalvanization [Japanese Patent Application Kokai (Laid-open) No. 58-181889], where the pH stability is improved by using phosphoric acid which is a weak acid, and a steel sheet with a comparatively better susceptibility to a phosphate salt treatment can be obtained, but there is an environmental pollution problem due to an inevitable discharge of waste phosphoric acid-based bath in the actual operation when the peeling bath is renewed.